Thermoelectric materials are materials that change temperature when an electric current is passed through them. Thermoelectric materials may be formed from a variety of semiconductor materials, and may be characterized in terms of the material's thermoelectric efficiency ZT or “figure of merit” at a temperature, which is defined by the relation:
      ZT    =                                        S            2                    ⁢          σ                k            ⁢      T        ,
Where S is the material's Seebeck coefficient; σ is the material's electrical conductivity; k is the material's thermal conductivity; and T is the temperature. The Seebeck coefficient is approximated as a voltage difference between two ends of a sample of thermoelectric material divided by a temperature difference between the two ends:
  S  =                    Δ        ⁢                                  ⁢        V                    Δ        ⁢                                  ⁢        T              .  
High Seebeck coefficients and electrical conductivity, coupled with low thermal conductivity, yield a high thermoelectric efficiency. Unfortunately, in many conventional thermoelectric materials, increases in Seebeck coefficient are associated with reduction in electrical conductivity and vice versa. Furthermore, electrical and thermal conductivity tend to be positively correlated, making it difficult to find materials having both a high electrical conductivity and a low thermal conductivity.
In underground drilling applications, such as oil and gas or geothermal drilling, a borehole is drilled through a subterranean formation, often deep into the earth. Such bore holes are drilled or formed by a drill bit connected to the end of a series of sections of drill pipe, so as to form an assembly commonly referred to as a “drill string.” The drill string extends from the surface to the bottom of the borehole. As the drill bit rotates under an applied axial force, commonly termed “weight on bit,” it advances into the earth, thereby forming the borehole. In order to lubricate the drill bit and flush cuttings from the drill bit's path as it advances, a high pressure solids-laden fluid, referred to as “drilling mud,” is directed through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through an annular passage formed between the exterior of the drill string and the surface or interior wall of the bore hole.
The distal or bottom end of the drill string, which includes the drill bit, is referred to as a “downhole assembly.” In addition to the drill bit, the downhole assembly often includes specialized modules or tools within the drill string that make up an electrical system for the drill string. Such modules often include sensing modules. In many applications, the sensing modules provide the drill string operator with information regarding the formation as it is being drilled through, using techniques commonly referred to as “measurement while drilling” (MWD) or “logging while drilling” (LWD). For example, resistivity sensors may be used to transmit and receive high frequency signals (e.g., electromagnetic waves) that travel through the formation surrounding the sensor.
As can be readily appreciated, such an electrical system may include many sophisticated electronic components, such as the sensors themselves, which in many cases include printed circuit boards. Additional associated components for storing and processing data in the control module may also be included on the printed circuit boards. Unfortunately, many of these electronic components generate heat. Even if the electronic component itself does not generate heat, the temperature of the formation itself, particularly in deep boreholes and in geothermal wells, may exceed the maximum temperature capability of the components (which may be about 150° C.
Overheating downhole frequently results in failure or reduced life expectancy for thermally stressed electronic components. Consequently, cooling of the electronic components can be important. Unfortunately, cooling is made difficult by the fact that the temperature of the formation surrounding deep wells, especially geothermal wells, is typically relatively high, and may exceed 200° C.
Downhole tools are exposed to tremendous thermal strain. The metal downhole tool housing is in direct thermal contact with the borehole drilling fluids and conducts heat from the borehole drilling fluid into the downhole tool housing. Conduction of heat into the tool housing raises the ambient temperature inside of the electronics chamber. Thus, the thermal load on a non-insulated downhole tool's electronic system is enormous and can lead to electronic failure. Electronic failure is time consuming and expensive. In the event of electronic failure, downhole operations must be interrupted while the downhole tool is removed from deployment and repaired.